GTR injectors will not fit... they are top feed, the SR needs side feed and they have a different impedance.
S15 injectors would be a good upgrade but I think you could only get them from Nissan and that would probably cost a bit but you could always give them a call and find out.
Series V RX7 13b injectors are another option. They're probably an overkill for what you need as they're 550cc.